The invention relates to a method of adapting a two-stage refrigerator cryopump to a specific gas; the cryopump includes a first cooling stage to which pump surfaces are fastened and which is equipped with a heating device; additionally the cryopump includes a second cooling stage to which pump surfaces are fastened and which, during operation, takes on a temperature between about 10 and 20 degrees K. The invention further relates to cryopumps suitable for implementing this method.
In cryopumps of this type, gases are captured primarily by employing the physical processes of "adsorption" and "condensation." Due to these processes, an unconditioned, two-stage refrigerator cryopump pumps without problems in all pressure ranges below 10.sup.-2 mbar, as long as the occurring gases can be grouped into three classes:
(a) adsorbable gases (H.sub.2, Ne, He) at T.ltoreq.20 degrees K. on adsorption surfaces;
(b) first condensable gases (N.sub.2, O.sub.2, Ar) at T.ltoreq.20 degrees K.;
(c) second condensable gases (typically: H.sub.2 O) at T.ltoreq.150 degrees K.
While for the operation of a second stage cryopump T.sub.2 .ltoreq.20 degrees K. is practically an operational requirement, the first stage is able to set itself within a broad range from about 50 degrees K. to 150 degrees K., depending on the size and type of the pump, the process and the external loads.
These circumstances are without a direct effect when pumping gases such as water vapor but may be of special importance for the occurrence of gases having vapor pressure curves between that of H.sub.2 O and that of O.sub.2 and N.sub.2. Examples for such gases are CO, N.sub.2 O, CH.sub.4, etc. The situation becomes particularly critical if these gases are present under varying pressure conditions (10.sup.-3 to 10.sup.-7 mbar). A gas particle entering the cryopump is condensed on its path within the cryopump at the first location which is just cold enough to bind the particle. From the vapor pressure curve of the respective gas it can be seen that, for example, a lower temperature is required to bind a gas at a pressure of .ltoreq.10.sup.-7 mbar than at a pressure of 10.sup.-3 mbar. Gases whose vapor pressure curves lie between the above-mentioned first condensable gases and the second condensable gases are therefore able to be condensed at an initially higher process pressure at sufficiently cold locations of the first stage. If one then desires to go toward lower pressures, this sometimes is not successful since the first stage is not cold enough for this purpose. At a pressure between 10.sup.-3 and 10.sup.-7 mbar, the gas that previously started to freeze in the first stage slowly travels over to the second stage, i.e. the pressure remains at an intermediate level; the pump no longer appears to be pumping.
European Patent No. 126,909 discloses the provision of a passive heat load for the pump surfaces of the first stage in that the outer surface of the radiation shield is blackened. Although the temperature of the first stage can be raised to a level which lies above a certain temperature by a passive load of this type, it is not possible to maintain a fixed temperature value. With increasing load on the pump, the temperature of the constantly loaded first stage which is relatively high in any case will rise to such an extent that it has a negative influence on the effectiveness of the pumping behavior. Moreover, the passive load cannot be changed so that a cryopump equipped with such a load may be suitable, for example, for pumping argon, but is no longer suitable for gases having higher vapor pressures. With such gases, the above-described rearrangements continue to occur. Another drawback of the passive load is that it is always present and thus extends the time required for the cryopump to reach the desired cold temperature.